Basically, refractive surgery involves a reshaping of the cornea to correct for optical aberrations. Although such reshaping can be accomplished in several ways, for purposes of the present invention it is envisioned that refractive surgery will be accomplished in accordance with protocols disclosed in the co-pending application for an invention entitled “Method for Intrastromal Refractive Surgery,” which is assigned to the same assignee as the present invention. The contents of this co-pending application are incorporated herein by reference.
In any surgical procedure, a preliminary diagnostic evaluation of the patient is essential. Moreover, for extremely complicated surgeries such as ophthalmic laser surgery, an accurate evaluation is essential for determining how the surgery should be accomplished. Further, and particularly with ophthalmic surgery, an evaluation helps determine the scope and extent of the surgery that is required. With so many variables involved, however, the ability to predict a surgical outcome with a high level of assurance can be extremely helpful.
As disclosed in the parent application, from which the present invention is a continuation, the use of a finite element model can be very helpful for predicting the outcome of an ophthalmic laser surgery procedure. Specifically, the finite element model disclosed in this parent application simulates a cornea and its response to a predetermined protocol for Laser Induced Optical Breakdown (LIOB) of stromal tissue in the cornea.
Every eye is unique and, accordingly, each eye has its own particular anatomical characteristics. Nevertheless, it happens that patients having similar vision defects will also have many similar anatomical characteristics in their respective corneas. Thus, in general, a finite element model may represent a corneal structure that exhibits a particular visual defect. Individualizing the model for a particular patient is then primarily a matter of scaling.
Further, a history of surgical treatments for a particular visual defect may produce a nomogram that indicates a particular LIOB protocol. Specifically, after performing LIOB on patients having essentially the same visual defect, the LIOB protocols and results may be analyzed and compiled to create a nomogram for future surgeries. The LIOB protocol indicated by this nomogram can be applied with a high degree of reliability for patients outside the group who have the same vision defect. This will be so, even though exact measures of corresponding values may be unknown. The consequence here is that a diagnostic nomogram which is characteristic of a surgical correction for a particular vision defect can be representative of a successful LIOB procedure for each member in an extended group of patients.
Zernike polynomials that mathematically model corneas having visual defects are given in the general form as:W(ρ,θ)=ΣcnmZnm(ρ,θ,αnm)
In the above expression, “n” pertains to the order of the polynomial (i.e. 2nd or 3rd order aberration) and “m” pertains to frequency (i.e. θ, 2θ, and 3θ). Further, cnm is a coefficient that pertains to magnitude; and Znm(ρ,θ,αnm) depends on radial and azimuthal considerations as they relate to a particular axis (αnm).
When considering the human eye as a genuine optical system, aberrations can be generally categorized as being either symmetric or asymmetric with respect to the optical axis of the eye. For this categorization, symmetrical aberrations are radially symmetrical with respect to the optical axis, while the asymmetrical aberrations are not. As indicated by the Zernike polynomials, in addition to their symmetry or lack thereof, the various optical aberrations of the eye can be categorized by their order. Insofar as imaging is concerned, it happens that the so-called lower order aberrations (i.e. 2nd, 3rd and 4th order) can be significantly detrimental. These lower order aberrations include both symmetrical and asymmetrical aberrations.
For purposes of the present invention, an appreciation for the interactive use of a particular model with Zernike polynomials for a finite element model is important. Specifically, it is known that a model can be created which will be representative of the cornea in all patients exhibiting a substantially same vision defect (e.g. presbyopia). Further, it is known that Zernike polynomials can be used to create the model. Using specific measurement values from a particular cornea, the Zernike polynomials can then be scaled to mathematically represent the optical condition of the particular cornea. Importantly, a model having this mathematical representation can then be used with a nomogram in a subsequent LIOB simulation. Further, the continuing modification of the model through LIOB simulation can lead to a desired corneal configuration. As a result, the necessary LIOB protocol to achieve the desired corneal configuration may be identified.
In light of the above, it is an object of the present invention to create a system and method for simulating a Laser Induced Optical Breakdown (LIOB) protocol to establish a surgical LIOB treatment for a patient. Another object of the present invention is provide a library of various nomograms and associated finite element models corresponding to respective visual defects for selected use in simulating LIOB procedures. Still another object of the present invention is to provide a system and method for simulating an LIOB procedure that is simple to use, easy to implement and cost effective.